1. Field of the Invention
This invention relates to a method and apparatus for heat treating molten glass in the forehearth of a glass furnace.
2. Description of the Prior Art
In the manufacture of glass articles such as bottles, molten glass is retained in the forehearth of a hollow glass furnace. When needed, the molten glass is withdrawn from the forehearth and delivered to several distribution conduits known as feeder conduits or "feeders". Parisons or gobs of glass pass from the feeders to forming machines which produce the desired glass article.
To prevent the glass from solidifying when the furnace is shut down, the forehearth is heated by gas burners. When the furnace is again operated, the power to the burners is increased to bring the entire mass of glass contained in the forehearth to a desired temperature.
Ideally, identical quantities of glass are drawn through each feeder and the average temperatures of the glass at the inlet of the various feeders are identical. In operation, however, the daily glass flow rates from each feeder differ from one another and their average inlet temperatures are different. In particular, the feeders having the lower flow rates also have the lower temperatures. Consequently, additional heat must be supplied to the molten glass contained in these feeders to bring the glass to a desired feeder output temperature. In the prior art, additional heating is effected by forehearth burners; but the glass entering these feeders still does not have a uniform temperature. Such thermal heterogeneities are detrimental because they cause substantial variations in the physical and chemical properties of the pieces manufactured from such glass.
When only one feeder is operating and its glass flow rate is low, the molten glass in the forehearth stagnates and cools to a temperature which is lower than the desired feeder output temperature. To maintain the desired output temperature, the prior art teaches one to overheat the entire mass of glass in the forehearth so that a small portion may be drawn from the operating feeder at the desired temperature. This is an excessively costly process. The prior art also teaches one to utilize burners aligned with the stream of molten glass passing through the forehearth and to the operating feeder. The burners heat the surface of the stream to a higher temperature than the glass beneath the surface. This, however, is unsatisfactory because the molten glass stream stratifies into layers having different temperatures, resulting in glass having undesirable properties.